1. Field of the Invention
This invention relates to a method and an apparatus for improving sharpness when recording half tone pictures, and is more particularly concerned with such a method and apparatus wherein optical/electrical scanning of picture patterns produce picture signals which are superimposed with a sharpness signal that is derived during scanning for increasing sharpness when recording.
2. Description of the Prior Art
Methods and apparatus are known in the art of reproduction technique whereby a single or a multi-colored half tone picture pattern is stretched as a top viewed picture, or as a translucent diapositive on a scanning drum, and is opto/electrically scanned. The scanning takes place in such a way that a dot-shaped light beam is directed onto the picture pattern to scan the picture pattern along the drum in a spiral-like manner by means of rotation of the drum and by means of an axial movement of the light beam. The light which is reflected or transmitted from the picture pattern is separated in multi-color printing by means of dichroic filters into the spectral regions of the basic colors cyan, magenta and yellow, and is transformed by photoelectric converters into electrical signals which constitute color separation signals. These color separation signals then control a recording member, such as an engraving needle or a recording lamp, by means of which the individual color separation signals are reproduced on a recording medium which is stretched on a recording drum. The movement of the recording drum, as well as the movement of the recording member, is synchronous with the scanning drum and with the scanning light beam so that the recording drum and the scanning drum rotate at the same speed or, if enlargement or reduction is desired, the recording speed is faster or slower than the scanning speed for reproduction. The same facts hold true for the relative axial movement of the scanning member with respect to the recording member, and thus to the recording medium.
Since the colors used during the printing process, the printing colors, are incomplete, and since also other parameters occur during the entire reproduction process which influence the final color of the reproduction, the color separation signals are subjected to one or several electronic treatments during transmission from the optical/electrical converter to the recording member, to nevertheless achieve the desired coloring during the printing process. These influences are generally gradation changes, color return and color correction.
Since the scanned picture pattern primarily constitutes a photographic picture which loses part of the sharpness contained in the original picture during development of a film, by subsequent enlargement or by optical printing, several attempts are made to regain the lost sharpness during the reproduction process. In addition to the color separation signals, a second picture signal is derived, which for an increase in sharpness, is again directed to the color recording medium. This picture signal will hereinafter be designated the sharpness signal.
Sharpness signals are provided, according to two principles. In one method, the picture signal which is derived during scanning of the picture pattern is deducted from the color separation signal. To achieve this result, the color separation signal is differentiated once or several times and the reslt in sharpness signal is subtracted from the color separation signal. The resulting signal is subsequently further processed as a sharpened color separation signal.
In another method, a so-called outer field is additionally scanned in addition to the scanning of a sharp point corresponding to the non-sharp masking in the photograph and reproduction technique. The signal which is achieved, and which constitutes the medium luminance of the outer field, is subtracted from the color separation signal. The difference signal is the sharpness signal and is added to the picture signal. A sharper signal is created which is correspondingly further processed during printing.
The method just-mentioned offers the advantage that the lost sharpness is regained; however, there is also the possibility that portions of the picture which were already relatively sharp in the original are reproduced with excessive sharpness. However, if the sharpness is reduced strongly, detailed portions of the picture, such as textiles and ornaments, are not reproduced with sufficient sharpness.